The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a technique, including a method and apparatus, for removing particle and residue build-up that occurs during substrate processing. The present invention is particularly useful for chemical vapor deposition processing, but may also be applied to plasma etching and other substrate processing techniques.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapor precursors. Such a deposition process is referred to as chemical vapor deposition or CVD. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to form a thin film layer over the surface of the substrate being processed. One particular thermal CVD application is the deposition of tungsten or a tungsten-containing compound, such as tungsten silicide, over a semiconductor substrate from a process gas that includes tungsten hexafluoride and dichlorosilane or silane, and/or hydrogen and other gases. Such a process is often used in deposition of a conductive layer, or as part of the deposition process of interconnects between one conductive layer and either the substrate or another conductive layer, or within one layer.
However, deposition occurs throughout the chamber, and not just on the substrate. The heaviest depositions occur in the hottest areas of the chamber, which is typically in the area of the substrate, but some deposition occurs in other areas, even fairly cool areas or areas not directly exposed to the vapor precursors.
These depositions can cause a number of problems. For example, they can clog fine holes in gas nozzles, disrupting an even flow of gas and affecting process uniformity. Or, they may cloud chamber windows, affecting the ability to see into the chamber. Additionally, they may form particulates, which can fall on the substrate to cause a defect in the deposited layer, or even interfere with the mechanical operation of the deposition system.
To avoid such problems, the inside surface of the chamber is cleaned regularly to remove the unwanted deposition material from the chamber walls and similar areas of the processing chamber. Such cleaning procedures are commonly performed between deposition steps for every wafer or every n wafers. One type of procedure involves disassembling the chamber and cleaning each part using a solution or solvent, then drying and reassembling the system. This procedure is labor-intensive and time-consuming, reducing wafer fabrication line efficiency and increasing costs.
Another common technique uses a plasma to promote excitation and/or dissociation of reactive gases by the application of radio frequency (RF) energy. In these techniques, a plasma of highly reactive species is created that reacts with, and etches away, the unwanted deposition material from the chamber walls and other areas. However, the plasma is generally contained within a bounded area of the chamber, and does not promote cleaning in areas outside of the plasma boundaries, such as the backside of a wafer holder, that may also require cleaning.
Another common problem with this technique is that many of the etchant gases employed in the plasma cleaning processes are perfluorocompounds or "PFC's", for short. Some of the more commonly used PFC's include CF.sub.4, CF.sub.6, NF.sub.3 and SF.sub.6 or similar gases. These gases are known to have a long lifetime (up to 50,000 years for CF.sub.4), and are also believed to degrade the earth's ozone layer. Thus, the release of PFC's into the atmosphere is potentially damaging and has become the subject of government and other regulations. Accordingly, it is important to reduce PFC emissions from semiconductor processing equipment such as CVD reaction chambers. Furthermore, a plasma-assisted cleaning process assumes that the chamber is configured to generate a plasma. Many chambers are not configured for plasma generation and cannot use this technique.
Molecular fluorine (F.sub.2) and chlorine trifluoride (ClF.sub.3) are examples of some non-PFC gases that have been employed as etchant gases in clean operations without plasma assistance. Molecular fluorine is a highly corrosive and dangerous gas that requires special measures, such as the use of double-walled tubing, to maintain proper safety levels. Diluted chlorine trifluoride has been used at relatively low temperatures around 300.degree. C. However, these low-temperature cleaning processes take a relatively long time. First, because the deposition processes typically take place at about 300.degree. C., the system must first be cooled down from, and then heated up to, the processing temperature. The greater the temperature difference is between the processing temperature and the cleaning temperature, the longer this takes. Also, because the cleaning rate increases with increasing temperature, a lower cleaning temperature requires a longer cleaning time for a fixed concentration of ClF.sub.3. Otherwise, a higher concentration of ClF.sub.3 must be used, which is more expensive and results in more waste products.
Because deposition typically occurs faster in the hotter areas of the chamber, and because the chamber temperature is not uniform, the build-up of deposits is not uniform throughout the chamber. Merely admitting a highly corrosive gas into the chamber for a period long enough to remove the heaviest deposits might overetch the areas with light deposits. While it is undesirable to overetch any chamber component, some chamber components, such as windows or fine nozzles, are especially vulnerable to attack by the etchant gases. The present cleaning processes utilizing F.sub.2 or ClF.sub.3 do not protect against overetching chamber components with light deposits. Overetching may result in compromised chamber performance and increased maintenance, which would decrease yields and throughput of the deposition system.